The intention of the milling-drying of raw brown coal is for the brown coal to be milled to a small particle size. At the same time, the intention is for moisture to be removed from the raw brown coal, such that brown coal dust with a low level of moisture is available after milling-drying has been effected. To this end, it is known in the prior art to dry the brown coal in a dryer (e.g. a tubular dryer) and to thereby remove the moisture from the brown coal. Then, the dried brown coal is milled to form brown coal dust and stored. In order to allay the risk of explosion during the milling operation and the risk of spontaneous ignition during storage, maximum temperatures which lie below the explosion or spontaneous ignition temperature are provided for the milling and the storage. In DE 42 23 151, 70 to 85° C. is indicated as the temperature range for milling, and a maximum temperature of 60° C. is indicated for storage. These temperatures can only be achieved by active cooling.
Furthermore, it is known to mill and dry brown coal in a single process step. To this end, the brown coal is dried during the milling operation, e.g. by injecting hot gas into the milling chamber of the mill. In order to avoid dust explosions in the mill, the drying gas must not exceed a certain temperature upon entry into the mill, however. VDI 2263 stipulates that the appropriate gas inlet temperature must not lie above ⅔ of the ignition temperature of the substance to be milled. For milling brown coal, this means a maximum gas inlet temperature of the drying gas of about 270° C. When it leaves the mill, the brown coal dust produced by a corresponding process is at a temperature of about 90° C., and has to be actively cooled to below 60° C. for safe storage.
In processes in which brown coal is milled and dried and then directly burned in a boiler, it is furthermore known to use flue gas for drying in the mill. In this case, the flue gas is at a temperature above the ignition temperature of brown coal, for example 1000° C. At the same time, however, this flue gas has a low oxygen content of about 6 to 12% by volume, and therefore dust explosions within the mill can be prevented. Since the brown coal dust is then directly burned, there is virtually no risk of a dust explosion and/or of spontaneous ignition of the brown coal dust.